1. Field of the Invention
The present invention relates to an image forming apparatus using an electro photo graphic process, such as a copying apparatus, a printer, a facsimile apparatus, a plotter, and a composite device thereof, and more particularly to a developing apparatus used in such an image forming apparatus.
2. Description of the Background Art
In accordance with recent demands for reductions in the size of this type of image forming apparatus for enhanced personal use, the size of the developing apparatus used in the image forming apparatus has also been reduced. These demands have also led to the wide use of apparatuses such as disposable developing apparatuses which are exchanged in their entirety when the toner runs out, and process cartridges in which the developing apparatus is integrated with a photosensitive body serving as a latent image carrier on which an electrostatic latent image of an original image is formed, a cleaning apparatus for removing residual toner from the photosensitive body, and so on. However, these small developing apparatuses are disadvantaged in that the amount of developer, comprising a toner and a carrier, that can be stored is small.
Moreover, the amount of space for a developer stirring unit for stirring the developer must also be reduced, and hence the amount of time required for replenishing toner to reach the developing area is shortened. As a result, the toner and magnetic carrier are not stirred sufficiently, leading to a reduction in the charge of the toner and the likelihood of floating toner. When floating toner is generated, surface staining of the transfer paper becomes striking, which is undesirable. This phenomenon occurs particularly when an original having a high image area ratio is printed continuously such that a large amount of replenishing toner is required and the replenishing toner is not dispersed and charged sufficiently.
Further, as image forming apparatuses increase in speed, the rotation speed of the developer stirring member also increases, and since the toner is constituted by minute particles of approximately 3 to 12 μm, a part of the replenishing toner is not stirred sufficiently with the developer, causing the toner to scatter, float, or slide along the surface of the developer. If the toner is then conveyed to the developing area by a developing roller, the uncharged, oppositely charged, or weakly charged toner causes staining on the non-image portion, density unevenness, and toner scattering.
To solve these problems, Japanese Unexamined Patent Application Publication H6-3950 (referred to as Prior Art 1 hereafter), for example, proposes a method of providing means for driving a stirring member independently such that the rotation speed of the stirring member can be varied independently during toner replenishment.
Further, Japanese Unexamined Patent Application Publication H10-198089 (referred to as Prior Art 2 hereafter) proposes a method of varying the stirring conditions of stirring means in accordance with the toner concentration level of the developer, which defines the relationship between an electrostatic charge distribution of the toner at the inlet to a developer stirring unit immediately after toner replenishment, and the amount of a weakly charged and/or oppositely charged toner component in the electrostatic charge distribution of the toner immediately before the outlet of the developer stirring unit following passage through the developer stirring unit.
Further, Japanese Unexamined Patent Application Publication H9-106161 (referred to as Prior Art 3 hereafter) proposes a technique of providing a scattering prevention member for preventing replenishing toner supplied from a toner replenishment unit from sliding over the surface of the developer and being conveyed uncharged to a screw portion on a developing roller side.
Further, Japanese Unexamined Patent Application Publication H11-202573 (referred to as Prior Art 4 hereafter) proposes a technique of raising the electrostatic charge by operating a developer path regulating member of a developing apparatus to approach a developing roller.
Further, Japanese Unexamined Patent Application Publication 2004-272017 (referred to as Prior Art 5 hereafter) proposes a constitution for varying the developer level between an upstream side and a downstream side of a replenishment side stirring chamber by providing more fins on the downstream side of a screw of the replenishment side stirring chamber than the upstream side.
However, with the techniques disclosed in Prior Art 1 and Prior Art 2, changes in the state of the developer caused by repetition of the image formation process are not taken into account, and hence the effects are insufficient. Following an investigation performed by the four present inventors and so on, it was found that following long-term repetition of the image formation process or in special use situations, for example when an original having a high image area is copied after image formation consuming a small amount of toner, such as continuous conveyance of white paper or an original having a low image area, has been performed over a long time period, the effects of Prior Art 1 and Prior Art 2 are not sustained.
Moreover, Prior Art 2 confirms that the behavior of the toner charge consists of both charge and discharge, and that the toner does not always change in one direction. Hence, the effects of Prior Art 2 may be insufficient according to use conditions and environmental conditions.
With the technique disclosed in Prior Art 3, of the two conditions that must be satisfied before the toner is conveyed to the developing unit, namely toner dispersal and toner charging, emphasis is placed on even toner dispersal. In other words, the screw portion is provided with a function for dispersing the replenishing toner into the developer evenly, while toner charging is handled by a conventional method of having the developer slide frictionally along a developer regulating blade. In this type of developing system, the toner receives a large load at a developer gathering portion on the rear side of the developer regulating blade, leading to deterioration of the developer. Deteriorated toner is likely to become weakly charged or oppositely charged, and hence when the image formation process is repeated over a long time period, the electrostatic charge distribution is broadened, leading to surface staining and toner scattering. However, when the stress on the rear of the developer regulating blade is simply reduced, the toner may not be charged sufficiently, causing even more surface staining and toner scattering.
With the technique disclosed in Prior Art 4, the frictional sliding force applied to the developer increases, but the developer regulating location is in the vicinity of a doctor blade, and when the toner is not completely dispersed before being conveyed to this position, concentration unevenness may occur despite the use of the developer regulating means.
All of the techniques disclosed in Prior Art 1 to Prior Art 5 are insufficient in preventing surface sliding of the replenishing toner, achieving even toner dispersal, performing toner charging adequately, and increasing the life of the developer by weakening the mechanical force that is applied conventionally to the developer. Investigations into the material constitution of developer are also currently underway to find ways of improving the charge buildup performance using various charge control agents and external additives. However, as the charge buildup performance improves, irregularities in the charge behavior are generated during toner replenishment.
Technologies relating to the present invention are also disclosed in, e.g. Japanese Unexamined Patent Application H7-175309, Japanese Unexamined Patent Application H10-149007, Japanese Unexamined Patent Application H10-198089, Japanese Unexamined Patent Application H11-202627, Japanese Unexamined Patent Application H11-231625, Japanese Unexamined Patent Application 2001-154471, and Japanese Unexamined Patent Application 2001-242688.